Armenians have marked 96 years since the mass killings of Armenians under the Ottoman Empire amid an apparent deadlock in the process of normalizing relations with Turkey.

Armenia says up to 1.5 million Armenians were systematically killed between 1915 and 1917, and describes the killings as genocide.

Turkey rejects the genocide label, and says 300,000 to 500,000 Armenians, but also at least as many Turks, died in civil strife that prevailed when the Ottoman Empire collapsed during World War I. The controversy has resulted in decades of hostility between the two countries.

In a written statement marking the anniversary of the killings, U.S. President Barack Obama said the mass deaths represent "one of the worst atrocities of the 20th century."

But for the third straight year, he stopped short of using the word genocide to describe the deaths.

In his statement, Obama said "contested history destabilizes the present and stains the memory of those whose lives were taken."

He praised efforts in Armenia and Turkey "to foster a dialogue that acknowledges their common history."

The Armenian Assembly of America, a U.S.-based group promoting U.S.-Armenian ties, described Obama's statement as a "missed opportunity."

Earlier in the Armenian capital Yerevan, several thousand rallied to demand Turkey recognize the killings as "genocide."

Last year, a U.S. Congressional panel gave its support to a statement brandishing the event as "genocide."

Turkey, however, reacted by recalling its ambassador from Washington and only sent him back after the statement failed to make it to a full vote in the House of Representatives.